Process for the preparation of solutions of urea-formaldehyde resins for the impregnation and coating of carrier webs for coating the surface of wood-based panels

Abstract

A method for the preparation of hardenable urea-formaldehyde resins for use in impregnating and coating carrier webs wherein urea and formaldehyde are first reacted in the presence of an aminosulfonic acid and ammonia, additional aminosulfonic acid is added and the mixture is further reacted to adjust the viscosity of the solution and then additional ammonia and urea are added to the reaction mixture and the mixture is further reacted to adjust the viscosity thereof, and wherein a polymer formed from the monomers of the formula ##STR1## in which R 1 =H or CH 3 , R 2 =O, R 3 =OH, NH 2 , OC n H 2n+1 wherein n is a whole number from 1 to 4, OC m H 2m OH wherein m is a whole number from 1 to 6, or OC m H 2m OC n H 2n+1 , or R 2 and R 3 are both N, is added in the form of an emulsion or dispersion. The product obtained by this process is particularly useful for coating wood-based panels in both short-cycle as well as multi-daylight presses.

What is claimed is: 
     
       1. A process for the preparation of solutions of hardenable urea-formaldehyde resins for impregnating and coating carrier webs for coating the surface of wood-based panels, in which an aqueous solution of: (a) urea and formaldehyde, in a molar ratio of 1:1.5 to 2.5 and in the presence of 0.2 to 1.0 mmoles of an amino-sulfonic acid and 20 to 100 mmoles of ammonia or 5 to 25 mmoles of urotropine, all amounts being based on 1 mole of urea, are reacted for 10 to 30 minutes at temperatures of 70° to 95° C. until the 50% solution has a viscosity of 55 to 65 cps at 20° C.; and then   (b) adding 0.8 to 10 mmoles of an aminosulfonic acid and maintaining the pH at a value between 4.0 and 4.5 with ammonia during a reaction time of 10 to 25 minutes at 70° to 95° C. until the 50% solution has a viscosity of 80 to 110 cps at 20° C., and finally;   (c) adding 40 to 200 mmoles of ammonia or 10 to 50 mmoles of urotropine and 0.1 to 0.3 moles of urea to the reaction product from step (b) and heating the reaction mixture at a temperature of 70° to 95° C. for 5 to 45 minutes until the 50% solution has a viscosity of 85 to 125 cps at 20° C.; and wherein 1 to 10 weight percent, based on the solid resin, of a polymer are added in the form of an emulsion or suspension to the reaction mixture in step (a), the polymer being formed from monomers of the formula ##STR3##  in which R 1  =H or CH 3 , R 2  =O,   R 3  =OH, NH 2 , OC n  H 2n+1  wherein n is a whole number from 1 to 4, OC m  H 2m  OH wherein m is a whole number from 1 to 6, OC m  OC n  H 2n+1 , or   R 2  and R 3  are both N.     
     
     
       2. The process of claim 1 wherein 1 to 10 weight percent, based on the solid resin, of a sulfonamide of the general formula   R.sup.4 SO.sub.2 NH.sub.2     are added to the reaction mixture, R 4  being an alkyl, or aryl or alkylaryl residue.   
     
     
       3. The process of claim 2 wherein the sulfonamide is added before or during step (a). 
     
     
       4. The process of claims 1 or 2 wherein 0.1 to 3.0 weight percent, based on the solid resin, of a latent hardening catalyst of the formula   R.sup.5 --SO.sub.3.sup.⊖.R.sup.6 NH.sub.3.sup.⊕     wherein R 5  is an alkyl, aryl or alkaryl residue and R 6  is an alkyl residue with 1 to 4 carbon atoms, are added to the solution of urea-formaldehyde resin.   
     
     
       5. A solution of a hardenable urea-formaldehyde resin prepared by the process of claim 1. 
     
     
       6. A carrier web coated with a solution of the hardenable urea-formaldehyde resin of claim 5.